The three effects that can change the flight path of an ink drop in an ink jet printer are charge repulsion between drops, charge induction between drops and aerodynamic drag. The ink drop is charged as it breaks off from the ink stream. This is typically accomplished by grounding the ink, which is conductive, and surrounding the ink stream at the drop breakoff point with a charge ring connected to some predetermined voltage. The voltage between the ink stream and the charge ring creates electrical charges in the ink stream which are trapped in the drop as the drop breaks off from the stream. The magnitude of this charge trapped on the drop is used to control the flight path of the drop by placing an electric field in the flight path to deflect the charged drop. Thus, a change in the voltage potential applied to the charge ring can change the charge in the drop and the flight path of the drop.
Charge induction errors in the flight path are caused by previously charged drops in the vicinity of the drop breakoff point inducing a charge on the drop currently breaking off. The charge placed on a drop is predominantly controlled by the charge ring but an error charge can be placed on the drop due to a previously charged drop near the drop breakoff point. The error in charging the drop then causes an error in the flight path of the drop to the print media.
The charge repulsion error effect is created by drops of the same charge repelling each other as they fly towards the print media. The repelling forces between the drops change their flight paths and thus change the point at which the drops strike the media creating an error in printing.
The aerodynamic drag on a drop can change the flight time of a drop to the print media. The faster the print media is moving relative to the drop stream, then the greater will be the errors in print position due to changes in flight time of a given drop. The amount of drag experienced by a drop depends upon the pattern of drops flying in front of the print drop or reference drop.
Each of the above three effects can create errors in precision ink jet printing. Which effect is dominant largely depends on the distance from the drop breakoff point to the print media and the relative velocity between the ink drops and the print media. If the velocity of the print media is slow relative to the ink drop velocity the predominant errors in printing are due to charge induction and charge repulsion. As the flight time of ink droplets increase and as the velocity of the print media relative to the droplets increase, aerodynamic drag becomes the more predominant source of error in printing. This is especially true in a binary ink jet system using uncharged drops as the print drops and charged drops as the gutter drops. Since the uncharged drops are the print drops the error effects due to induced charges and charge repulsion are small compared to the errors due to the aerodynamic drag on the drops.
In addition, the error effect of induced charges or charge repulsion is limited to substantially the three or four drops immediately in the vicinity of the reference drop. It is known for example that the charge induction effect falls off nonlinearly with distance from the reference drop (drop breaking off). The fourth drop away from the reference drop is the last drop that usually needs to be considered (for example, see U.S. Pat. No. 4,032,924, issued to Takano et al on June 28, 1977). Similarly, the charge repulsion effect between drops decreases as an inverse function of the squared distance between the drops. Thus, the charge repulsion effect on print error need be considered only for drops immediately in the vicinity of the reference drop.
On the other hand, the aerodynamic error effect, when it is predominant has been found to be a long term effect. In some situations drops in excess of 30 drop positions in front of the reference drop can have an effect on the aerodynamic drag on the reference drop.
Examples of apparatus compensating for induced charges are taught in U.S. Pat. Nos. 3,631,511 and 3,789,422. The Keur et al U.S. Pat. No. 3,631,511 issued on Dec. 28, 1971, teaches correcting the reference drop for induced charge from the immediately preceeding drop. The Haskell et al U.S. Pat. No. 3,789,422 issued Jan. 29, 1974, teaches compensating for charge effects based upon any number of previously charged drops.
U.S. Pat. Nos. 3,828,354 and 3,946,399 teach compensating for the error effects due to charges and aerodynamic drag. The Zareski U.S. Pat. No. 3,946,399 issued on Mar. 23, 1976, teaches monitoring the data pattern for an ink jet stream to detect particular print data patterns. These print data patterns are then logically analyzed to select a compensation charge signal to be applied to the charge ring. The Hilton U.S. Pat. No. 3,828,354 issued on Aug. 6, 1974, teaches monitoring a seven bit print data pattern to generate the compensation signal for aerodynamic and charge induced effects. Hilton monitors four drops ahead of the reference drop two drops behind the reference drop and the reference drop itself. Based upon the binary pattern for these seven drops, Hilton addresses a read-only-store memory which contains predetermined compensation values for each possible address.
None of the above patents teach compensating for the relatively long term aerodynamic drag effects. One problem in trying to correct for such effects is the number of patterns to be corrected for. If drops as far as 30 drop positions away from the reference drop have an effect, then the number of possibilities requiring correction are 2.sup.30. Clearly storing a charge compensation value for each and every possibility is not practical.
The basic solution to the above problem is to compensate the reference drop for each and every drop in the immediate proximity to the reference drop and to summarize the effect of groups of drops more remote from the reference drop. Embodiments of this solution to the problem are shown in FIGS. 1, 2, and 3 herein and claimed in related application, Ser. No. 23,813, filed Mar. 26, 1979 now U.S Pat. No. 4,229,749. However, further improvement of print quality can be achieved with the same limited memory space if tradeoffs are made between print data patterns taking into account the print error distribution produced by the pattern combinations.